Coupling elements are known that comprise a body in which a housing is provided for receiving the tube end. The housing has an inlet segment into which the tube end penetrates, and a welding segment having an inlet that is defined by a surface for initiating welding.
The tube end is secured to the coupling by a friction-welding method comprising the steps of:                inserting the tube end in the housing;        causing the tube end to rub against the weld-initiating surface in order to soften the material of the tube and the material of the element; and        inserting the tube end into the welding segment of the housing in order to weld the tube end therein.        
That technique for securing the tube end to the coupling element is particularly fast and effective with tubes and coupling elements conventionally made out of a single layer of thermoplastic material.
Nevertheless, the thermoplastic materials constituting tubes and the bodies of coupling elements are more and more frequently incorporating fillers seeking to improve their performance and in particular their ability to withstand mechanical stresses, chemical substances, high temperatures, radiation . . . Unfortunately, there is a risk that such fillers constitute non-uniform zones in a weld, thereby weakening the weld.
In addition, the operating stresses to which fluid transport tubes are subjected make it necessary to have recourse to multilayer structures in which each layer constitutes an economic response to one or more of the constraints to which the tube is subjected. By way of example, a multilayer tube may comprise an inner layer that is chemically compatible with the fluid to be conveyed, an intermediate layer that forms a gas barrier and/or that performs a mechanical reinforcement function, and an outer layer that withstands external attack such as high temperatures, ultraviolet radiation, chemical attack, . . . . Only the outer layer or the inner layer is made of a material that is suitable for welding to the material constituting the coupling element. As a result, the thickness of the material available for welding purposes is relatively small compared with the thickness available in a single-layer tube in which a greater thickness can be used for providing the weld. The stresses that result from a traction force being exerted on the tube are therefore concentrated in a relatively small zone, and that can lead to the weld being damaged.
Document U.S. Pat. No. 4,547,239 discloses an element for securing to a tube end by friction welding. The element has a housing for receiving the tube end. The housing has an inlet segment connected to a welding segment via a weld-initiating surface. A helical groove is formed in the welding segment. During welding, the material constituting the end of the tube expands into the groove. Nevertheless, retention of the tube runs the risk of being unsatisfactory in the event of strong forces acting on the tube.